In Add Notes mode notes can be added by leftclicking the mouse. The dropdown list next to the Add Notes mode button determines the duration of notes being added. Keeping the S or F key down while clicking the mouse adds or subtracts a semitone ('Sharp' or 'Flat'), so you can add notes that are not in the current scale directly.
The b and # buttons can be used to transpose the selected notes one semitone down or up. Right clicking a note pops up a menu to (among other things) join the note with the previous note.
The Score editor has a resolution of 1/32nd note. It snaps to the grid automatically, regardless of the Snap setting. Although it is possible to use the Score editor when the time scale is set to seconds, this obviously doesn't make sense.
Triplets, quintuplets and septuplets are recognized automatically. Tuplets can be added by right clicking the editor, and picking one out of the items in the menu's Add Tuplet section. Alternatively you can add a note and use the 'More...' button's 'Split in equal parts' option.
The Key Signature is a value ranging from 7 flats up to 7 sharps. Key signature changes throughout the song can be programmed after clicking the Edit Changes link. Key signature changes always occur at the start of a bar. When any changes are present the left hand section shows the key signature of the first bar that's (partially) visible. The key signature settings affect all MIDI tracks. It is stored in the MIDI files.